1. Field of the Invention
The present invention relates to an array antenna apparatus, mainly for mobile communication, having at least two feeding elements and operable in multiple frequency bands, and relates to a wireless communication apparatus provided with this array antenna apparatus.
2. Description of the Related Art
The size and thickness of portable wireless communication apparatuses, such as mobile phones, have been rapidly reduced. Portable wireless communication apparatuses have been transformed from apparatuses to be used only as conventional telephones, to data terminals for transmitting and receiving electronic mails and for browsing web pages of WWW (World Wide Web), etc. Further, since the amount of information to be handled has increased from that of conventional audio and text information to that of pictures and videos, a further improvement in communication quality is required. In such circumstances, an array antenna apparatus provided with multiple antenna elements, and an antenna apparatus capable of switching among directivities have been proposed.
PCT International Publication WO02/39544 discloses an antenna device including a rectangular conductive board, and a flat plate antenna mounted on the board with a dielectric interposing therebetween. The antenna device is characterized by exciting the antenna in a certain direction so as to flow a current through the board in one diagonal direction, and exciting the antenna in a different direction so as to flow a current through the board in the other diagonal direction. As such, in the antenna device disclosed in PCT International Publication WO02/39544, the directivity and polarization direction of the antenna device can be changed by varying the direction of a current flowing through the board.
Japanese Patent Laid-Open Publication No. 2005-130216 discloses a mobile radio apparatus that is foldable and that has a mechanism joining a first case and second case at a hinge part allowing said mobile radio apparatus to open and close. The mobile radio apparatus includes: a first flat conductor placed on a first plane inside the first case along a longitudinal direction of the first case, and second and third flat conductors placed on a second plane opposing a first plane inside the first case along the longitudinal direction of the first case, and feeding means for feeding the first flat conductor and feeding selectively the second or the third flat conductor at a phase different from a phase with which the first flat conductor is fed. The mobile wireless apparatus disclosed in Japanese Patent Laid-Open Publication No. 2005-130216 can improve communication performance by switching between the second and third flat conductors in response to a reduction in reception level.
PCT International Publication WO01/97325 discloses a portable radio unit including a dipole antenna, and two feeding means each connected to one of two antenna elements that compose the dipole antenna.
Recently, an antenna apparatus has appeared that adopts MIMO (Multi-Input Multi-Output) technology for simultaneously transmitting and/or receiving radio signals of a plurality of channels by space division multiplexing, in order to increasing communication capacity and achieve high-speed communication. The antenna apparatus that performs MIMO communication needs to simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other, each having a different directivity, polarization characteristics, or the like, in order to achieve the space division multiplexing. The antenna device disclosed in PCT International Publication WO02/39544 can switch over to a different directivity, however, this antenna device cannot simultaneously implement a plurality of states, each having a different directivity. The mobile radio apparatus disclosed in Japanese Patent Laid-Open Publication No. 2005-130216 requires a plurality of antenna elements (flat conductors), and results in a complicated structure. Furthermore, in a similar manner to that of the antenna device disclosed in PCT International Publication WO02/39544, although this mobile radio apparatus can switch over to a different directivity, this mobile radio apparatus cannot simultaneously implement a plurality of states, each having a different directivity. The portable radio unit disclosed in PCT International Publication WO01/97325 cannot switch between directivities, and also cannot simultaneously implement a plurality of states, each having a different directivity.
Moreover, in the case that an array antenna is mounted in a small-sized wireless communication apparatus such as a mobile phone, a distance between feeding elements is inevitably reduced, thus resulting in a problem that isolation between the feeding elements becomes insufficient.
Additionally, it is desirable to provide an antenna apparatus operable in multiple frequency bands, as well as capable of the MIMO communication, in order to perform, e.g., communications for a plurality of applications. Such an antenna apparatus has not been disclosed in any of PCT International Publication WO02/39544, Japanese Patent Laid-Open Publication No. 2005-130216, and PCT International Publication WO01/97325.